A conventional electric power steering apparatus assists a steering force applied by a driver to a steering wheel by detecting an input torque applied to a steering shaft using a torque sensor, and controlling driving of an electric motor on the basis of a detection result.
In this type of electric power steering apparatus, two torque sensor systems are provided, and when a difference of at least a predetermined value appears between output values of the two torque sensor systems, it is determined that a breakdown has occurred in the torque sensor.
In an apparatus provided with two torque sensor systems, however, when both systems are disconnected, the output values of the two torque sensor systems both correspond to a neutral point in which no steering torque is input, and therefore, with the above method of determining a breakdown on the basis of the difference between the output values of the two systems, it is impossible to determine that a breakdown has occurred in both torque sensor systems. In this type of situation, the output value of the torque sensor corresponds to the neutral point in which no steering torque is input even when the driver operates the steering wheel such that steering torque is generated in the steering shaft, and therefore assistance is not provided by the electric motor. As a result, the steering wheel becomes heavy.
JP2008-62686A discloses an electric power steering control apparatus that determines that an abnormality has occurred in a torque sensor when a condition in which two output values obtained from a torque sensor are in the vicinity of a reference value and a steering angle of a steering wheel equals or exceeds a predetermined angle remains established for a predetermined time.